Documentation is the first thing to add. If Codex is touching a package, framework, or cloud SDK, having current docs available is the highest-value external context you can give it.
After that, add repository workflow context if tasks start from issues, reviews, or failing CI runs. Add database access when the code genuinely depends on schema reality rather than application-level assumptions. Hold off on memory tools until you have actual recurring decisions worth storing.
Codex does not need a large MCP surface area to do good work. A small, reliable set of servers is also easier to review when an agent takes an unexpected action — which matters more than people expect.